1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, etc. More specifically, the present invention is preferably applied to an image forming apparatus of the cleaner-less type where developer (toner) remaining on an image bearing member after a transfer process is removed and collected for reuse simultaneously by development and cleaning in a developing device, thus eliminating a cleaning device.
2. Description of the Related Art
Image forming apparatus using transfer type electrophotography such as copying machines, printers, and facsimile machines have hitherto been known. Such an image forming apparatus generally comprises a photosensitive member as an image bearing member which is generally a rotating drum, a charging device for uniformly charging the photosensitive member in a predetermined polarity and at a predetermined potential (charging process), an electrostatic latent image forming device, that is, an exposure device for forming an electrostatic latent image on the charged photosensitive member (exposing process), a developing device for visualizing by developer (toner) the electrostatic latent image formed on the photosensitive member (developing process), a transfer device for transferring the toner image from the photosensitive member onto a transfer material such as paper (transfer process), a cleaning device for removing a little amount of toner remaining on the photosensitive member after the transfer process to clean the surface of the photosensitive member (cleaning process), a fixing device for fixing the toner image on the transfer material (fixing process), and the like. The photosensitive member is repeatedly used in image formation in the electrophotographic process (the charging process, the exposing process, the developing process, the transfer process, and the cleaning process).
The toner remaining on the photosensitive member after the toner image is transferred onto the transfer material is removed from the surface of the photosensitive member by the cleaning device. The removed toner is collected by and in the cleaning device. The collected toner accumulates in the cleaning device as waste toner. However, from the viewpoint of environment conservation, effective use of resources, and the like, it is preferable that no waste toner is generated.
Thus, there have been known image forming apparatus which return waste toner accumulated in the cleaning device to the developing device to reuse the toner in the developing device
Cleaner-less image forming apparatus, which eliminate the cleaning device, are also known. A cleaner-less image forming apparatus removes toner remaining on a photosensitive member by the developing device after the transfer process, so that the removed toner is collected in the developing device and reused in the developing device. In such a cleaner-less image forming apparatus, the developing device cleans up the toner remaining on the photosensitive member simultaneously with development of an electrostatic latent image (cleaning simultaneous with developing)
The cleaning simultaneous with developing is a method in which residual toner, that is, toner which is not transferred by the transfer device and is remaining on the photosensitive member, is collected in the developing device in following developing processes. More specifically, the photosensitive member having residual toner thereon is subsequently charged by a charging device, and an electrostatic latent image is formed on the surface of the photosensitive member by an exposure device, whereafter the electrostatic latent image is developed by the developing device. Simultaneously with the development of the electrostatic latent image by the developing device, part of the toner on a non-exposed portion among the toner remaining on the photosensitive member without being developed is collected in the developing device by application of a fog removal bias voltage (fog removal potential difference Vback in the form of a potential difference between the direct current voltage applied to the developing device and the surface potential of the photosensitive member).
According to this method, the residual toner is collected in the developing device and used in following developing processes. Thus, no waste toner is generated, and troublesome maintenance for collecting waste toner is eased. Further, since the cleaning device is unnecessary, this method is also effective in reducing the size of the image forming apparatus.
However, in case where the charging device comprises a contact charging device which is in contact with the surface of the photosensitive member to charge the photosensitive member, when residual toner on the photosensitive member passes through contact portions (charging portion) of the photosensitive member and the contact charging device, part of the residual toner (reversal toner), which is charged to a polarity opposite to a normal polarity of toner, attaches to the contact charging device. As a result, the contact charging device is contaminated by toner to an unacceptable level, and hence the charging device becomes unable to sufficiently charge the photosensitive member.
In view of these circumstances, the present inventors proposed an image forming apparatus of the cleaner-less type which, when the charging device is a contact charging device, is free from defective charging and a poor image by preventing residual toner from attaching to the charging device and by efficiently collecting the residual toner by means of the developing device, and which offers benefits of a cleaner-less image forming system (see U.S. Pat. No. 6,421,512).
The above-mentioned image forming apparatus includes a first developer charge amount control device (first developer charging member) arranged on the downstream side of a transfer device in a rotational direction of a photosensitive member for charging residual developer on the photosensitive member, and a second developer charge amount control device (second developer charging member) arranged on the downstream side of the first developer charge amount control device and on the upstream side of the charging device for charging the residual developer on the photosensitive member. The first developer charge amount control device charges the residual developer, that is, the developer which is remaining on the photosensitive member without being transferred by the transfer device, to a polarity opposite to the normal polarity of the developer (toner). Then, the second developer charge a mount control device charges the residual developer, which was charged to the opposite polarity to the normal polarity of the developer, to the normal polarity. Thereafter, the charging device charges the photosensitive member, and at the same time, appropriately charges the residual developer.
As a result, the residual developer is prevented from attaching to the charging device, and the developing device efficiently collects the residual developer, whereby it is possible to provide an image forming apparatus which is free from defective charging and a poor image while offering benefits of a cleaner-less image forming system.
However, in case where a developer charge amount control device is in contact with a photosensitive member, a some small amount of developer may remain on the contact (nip) portions of the developer charge amount control device and the photosensitive member. At the instant when a bias voltage is applied to the developer charge amount control device, and/or at the instant when the application of a bias voltage is stopped, the residual developer no longer stays on the contact portions of the photosensitive member and the developer charge amount control device, and is transferred onto the photosensitive member, thus resulting in formation of a poor image. In order to prevent such a situation, U.S. Publication No. 2003-59231 proposes that the slope of a bias voltage applied be defined appropriately.
Even if, however, an appropriate bias voltage is applied so as to prevent the transfer of the residual developer (toner) to the photosensitive member, when the amount of the toner remaining on the contact portions of the photosensitive member and the developer charge amount control device exceeds a prescribed value, the developer charge amount control device is contaminated by part of the toner of high resistance, resulting the cause of defective charging.